1. aromaticity2

2. Cyclobutadiene: Square or Rectangular?
The ground-state structure of 77 is a rectangular diene (not a diradical) as shown by the ir spectra of 77 and deuterated 77 trapped in matrices, as well as by a photoelectron spectrum.
Molecular-orbital calculations agree the same conclusion was also reached in an elegant experiment in which 1,2-dideuterocyclobutadiene was generated.
If 77 is a rectangular diene, the dideutero compound should exist as two isomers

3. Stable Cyclobutadiene
Some simple cyclobutadienes are stable at room temperature for varying periods of time.
They have bulky substituents or carry certain other stabilizing substituents
Such compounds are relatively stable because dimerization is sterically hindered.
Examination of the NMR spectrum of 83 showed that the ring proton (δ=5.38) was shifted upfield, compared with the position expected for a nonaromatic proton.

4. Push–Pull or Captodative Effect
The other type of stable cyclobutadiene has two electron-donating and two electron-withdrawing groups.
The stability of these compounds is attributed to a type of resonance stabilization called the push–pull or captodative effect.
Although it has been concluded from a photoelectron spectroscopy study that second-order bond fixation is more important

5. Systems of Four Electrons: Antiaromaticity
Cyclopropenyl Anion Cyclopentadienyl Cation
With respect to 86, HMO theory predicts that an unconjugated 85 is more stable than a conjugated 86.
So that 85 would actually lose stability by forming a closed loop of four electrons.
The HMO theory is supported by experiment

6. Cyclopropenyl Anion Cyclopentadienyl Cation
88 (R = COPh) loses its proton in hydrogen-exchange reactions 6000 times more slowly than 89 (R = COPh).
When 90 is treated with silver perchlorate in propionic acid, the molecule is rapidly solvolyzed.
Under the same conditions, 92 undergoes no solvolysis at al

7. Systems of Eight Electrons: Antiaromaticity
[8]annulene is tub-shaped. Therefore we would expect that it is neither aromatic nor antiaromatic.
The reason for the lack of planarity is that a regular octagon has angles of 135, while sp2 angles are most stable at 120.
To avoid the strain, the molecule assumes a nonplanar shape, in which orbital overlap is greatly diminished.
Single- and double-bond distances in are respectively, 1.46 and 1.33A°
The reactivity is also what would be expected for a linear polyene. Reactive intermediates can be formed in solution.

8. Systems of Eight Electrons: Antiaromaticity
The 93 and 94 are planar conjugated eight-electron systems (the four extra triple-bond electrons do not participate), which nmr evidence show to be antiaromatic.
There is evidence that part of the reason for the lack of planarity in cyclooctadiendiynes itself is that a planar molecular would have to be antiaromatic.

9. Systems of Ten Electrons
The all-cis (97), the mono-trans (98), and the cis–trans–cis–cis–trans (79).
It is far from obvious that the molecules would adopt a planar shape, since they must overcome considerable strain to do so.
For a regular decagon (97) the angles would have to be 144, considerably larger than the 120 required for sp2 angles.
Some of this strain would also be present in 98, but this kind of strain is eliminated in 79 since all the angles are 120.
The hydrogens in the 1 and 6 positions of 79 should interfere with each other and force the molecule out of planarity. (Mislow Effect)
These results indicate that [10]annulene is sufficiently distorted from planarity that little aromatic stabilization is achieved.

10. Systems of Ten Electrons
Among these are the dianion 99, the anions 100 and 101, and the azonine 102.
Compound 99 has angles of 135, while 100 and 101 have angles of 140, which are not very far from 144.
The inner proton in 101 (which is the mono-trans isomer of the all-cis 100) is found far upfield in the NMR (-3.5 δ).

11. Systems of Ten Electrons
The 103,104 and 105, have been prepared they undergo aromatic substitution and are diatropic.
The protons of 103 are found at 6.9–7.3 δ, while the bridge protons are at -0.5 δ.
The 103 is nonplanar, but the bond distances are in the range 1.37–1.42A°
A small distortion from planarity does not prevent aromaticity

12. Systems of Ten Electrons
In 106, no canonical form can be written in which both benzene rings have six electrons, the aromaticity is reduced by annellation.
The 106 rapidly converts to the more stable 107, in which both benzene rings can be fully aromatic

13. Systems of More than Ten Electrons 4n+2 Electrons (14e)
Compound 109 is aromatic (it is diatropic; inner protons at 0.00 δ, outer protons at 7.6 δ).
It is completely destroyed by light and air in 1 day. X-ray analysis shows that although there are no alternating single and double bonds, the molecule is not planar.
Mislow Effect

14. Systems of More than Ten Electrons;4n+2 Electrons (14e)
The 110 is aromatic approximately planar; the bond distances are all 1.39–1.40A ° ; molecule undergoes aromatic substitution and is diatropic.
The outer protons found at 8.14–8.67 δ, while the CH3 protons are at δ.
Annulenes 111 and 112 are diatropic, although X-ray crystallography indicates that the π periphery in at least is not quite planar.
The 113, the geometry of the molecule reduces the overlap of the π orbitals at the bridgehead positions with adjacent p orbitals, is not aromatic.

15. Systems of More than Ten Electrons; 4n+2 Electrons (14e)
Another way of eliminating the hydrogen interferences of [14]annulene is to introduce one or more triple bonds into the system, as in 114 and 116.
The extra electrons of the triple bond do not form part of the aromatic system exist as a localized bond.

16. Systems of More than Ten Electrons;4n+2 Electrons (18e)
[18]Annulene (115) is diatropic; the 12 outer protons are found at δ=9 and the 6 inner protons at δ=-3. It is nearly planar, so that interference of the inner hydrogens is not important in annulenes this large. Compound 115 is being distillable at reduced pressures, and undergoes aromatic substitutions. The C=C bond are not equal, but they do not alternate. There are 12 inner bonds of 1.38A° and 6 outer bonds of 1.42A°. Compound 115 has been estimated to have a resonance energy of 37 kcal mol1 similar to that of benzene.

17. Systems of More than Ten Electrons; Kekulene; Superaromatic
The 1H NMR spectrum of 118 shows three peaks at δ =7:94, 8.37, and in a ratio of 2:1:1.
The peak at 7.94δ is attributed to the 12 ortho protons and the peak at 8.37 δ to the six external para protons. The remaining peak comes from the six inner protons.
If the molecule preferred 118b, we would expect to find this peak upfield, probably with a negative δ, as in the case of 115

18. Systems of More than Ten Electrons; Kekulene; Superaromatic
Phenacenes are a family of ‘‘graphite ribbons,’’ where benzene rings are fused together in an alternating pattern.
From benzene to heptacene, reactivity increases although acene resonance energies per p electron are nearly constant.
The inner rings of the ‘‘acenes’’ are more reactive, and calculations shown that those rings are more aromatic than the outer rings, and even more aromatic than benzene itself.
26-electron fulminene
22-electron picene
30 electron phenancene

19. Systems of More Than Ten Electrons 4n Electrons
In solution, the [12]annulene 124 undergoes rapid conformational mobility and above -150C in this partiuclar case, all protons are magnetically equivalent.
However, at -170C the mobility is greatly slowed and the three inner protons are found at 8 δ while the nine outer protons are at 6 δ.
Interaction of the ‘‘internal’’ hydrogens in 124 leads to nonplanarity. Above -50C, 124 is unstable and rearranges to 125.

20. Systems of More Than Ten Electrons 4n Electrons 12e
The NMR spectra show that all four compounds are paratropic, the inner proton of 126 being found at 16.4 δ.
In these compounds, both hydrogen interference and conformational mobility are prevented.
In 127–129, the bridge prevents conformational changes, while in 126 the bromine atom is too large to be found inside the ring.

21. Systems of More Than Ten Electrons 4n Electrons 16e
The 131 in solution is in equilibrium with 132 and above 50C there is conformational mobility, resulting in the magnetic equivalence of all protons.
At 130C the compound is paratropic: there are 4 protons at δ and 12 at 5.35 δ.
In the solid state, the molecules are nonplanar with almost complete bond alternation: the single bonds are 1.44–1.47A° and the double bonds A° .

22. Systems of More Than Ten Electrons 4n Electrons
(133) and (134) are paratropic, as shown by NMR. These molecules might have been expected to behave like naphthalenes with outer bridges, but the outer π frameworks constitute antiaromatic systems with an extra central double bond.
With respect to 133, the 4n+2 rule predicts 133 to be ‘‘aromatic’’ if it is regarded as a 10π electron naphthalene unit connected to two 2 π electron etheno systems, but ‘‘antiaromatic’’ if it is viewed as a 12π electron periphery perturbed by an internal cross-linked etheno unit.
Recent studies have concluded on energetic grounds that 133 is a ‘‘borderline’’ case, in terms of aromaticity–antiaromaticity character. Dipleiadiene appears to be antiaromatic.

23. Systems of More Than Ten Electrons 4n Electrons
In 110, the outer protons were found at 8.14–8.67δ with the methyl protons at δ.
For the dianion, which has 16 electrons, the outer protons are shifted to about -3 δ while the methyl protons are found at 21 δ, a shift of 25 δ!
We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic.

24. Huckel–Mobius Concept
The concept of ‘‘Mobius aromaticity’’ was conceived by Helbronner in 1964 when he suggested that large cyclic 4n annulenes might be stabilized if the p-orbitals were twisted gradually around a Mobius strip.
This concept is illustrated by the diagrams labeled Huckel, which is a destabilized 4n system, in contrast to the Mobius model, which is a stabilized 4n system.

25. Other Aromatic Compounds
1. Mesoionic Compounds
These compounds cannot be satisfactorily represented by Lewis structures not involving charge separation.
Most of them contain five-membered rings. The most common are the sydnones, stable aromatic compounds that undergo aromatic substitution when R1 is hydrogen.

26. Other Aromatic Compounds
2. The Dianion of Squaric Acid
The stability of this system is illustrated by the fact that the pK1 of squaric acid is 1.5 and the pK2 is 3.5,which means that even the second proton is given up much more readily than the proton of acetic acid.

27. Other Aromatic Compounds
3. Homoaromatic Compounds.
In 137, an aromatic sextet is spread over 7 carbons, as in the tropylium ion. The eighth carbon is an sp3 carbon and so cannot take part in the aromaticity.
NMR show the presence of a diatropic ring current: Hb is found at δ=-0.3; Ha at 5.1 δ; H1 and H7 at 6.4 δ; H2–H6 at 8.5 δ. This ion is an example of a homoaromatic compound, which may be defined as a compound that contains one or more sp3-hybridized carbon atoms in an otherwise conjugated cycle.

28. Other Aromatic Compounds
4. Fullerenes
Fullerenes are a family of aromatic hydrocarbons based on the parent buckminsterfullerene (138; C60) that have a variety of very interesting properties.
Molecular-orbital calculations showed that ‘‘fullerene aromaticity lies within 2 kcal mol-1 (8.4 kJ mol-1) per carbon of a hypothetical ball of rolled up graphite.
Another class of polynuclear aromatic hydrocarbons are the buckybowls, which are essentially fragments of 138.
Corannulene (139) is the simplest curved-surface hydrocarbon possessing a carbon framework that is identified with the buckminsterfullerene

29. Fullerenes
Kroto, H.W.; Heath, J.R.; O’Brien, S.C.; Curl, R.F.; Smalley, R.E. Nature, 1985, 318, 162.

30. TAUTOMERISM
For most compounds, all the molecules have the same structure, whether or not this structure can be satisfactorily represented by a Lewis formula.
But for many other compounds there is a mixture of two or more structurally distinct compounds that are in rapid equilibrium.
When this phenomenon, called tautomerism, exists, there is a rapid shift back and forth among the molecules. In most cases, it is a proton that shifts from one atom of a molecule to another

31. Keto–Enol Tautomerism
A very common form of tautomerism is that between a carbonyl compound containing an a hydrogen and its enol form: Such equilibria are pH dependent,

33. Main Types of the More Stable Enols
Molecules in which the enolic double bond is in conjugation with another double bond.
Molecules that contain two or three bulky aryl groups.
Highly fluorinated enols

34. Other Proton-Shift Tautomerism
1. Phenol–Keto Tautomerism 2. Nitroso–Oxime Tautomerism 3. Aliphatic Nitro Compounds Are in Equilibrium with Aci Forms 4. Imine–Enamine Tautomerism 5. Ring-Chain Tautomerism

35. Phenol–Keto Tautomerism
For most simple phenols, this equilibrium lies well to the side of the phenol, since only on that side is there aromaticity.
For phenol itself, there is no evidence for the existence of the keto form.

36. Phenol–Keto Tautomerism
keto form becomes important and may predominate when:
(1) where certain groups, such as a second OH group or an NO group, are present.
(2) in systems of fused aromatic rings.
(3) in heterocyclic systems.

37. Nitroso–Oxime Tautomerism
In molecules where the products are stable, the equilibrium lies far to the right, and as a rule nitroso compounds are stable only when there is not a hydrogen

38. Aliphatic Nitro Compounds Are in Equilibrium with Aci Forms
The nitro form is much more stable than the aci form in sharp contrast to the parallel case of nitroso–oxime tautomerism, undoubtedly because the nitro form has resonance not found in the nitroso case.
Aci forms of nitro compounds are also called nitronic acids and azinic acids.

39. Imine–Enamine Tautomerism
Enamines are normally stable only when there is no hydrogen on the nitrogen (R2C=CR-NR2). Otherwise, the imine form predominates.
The energy of various imine–enamine tautomers has been calculated.

40. Ring-Chain Tautomerism

41. M. Gorlitz and H. Gunther, Tetrahedron, 25, 4467 (1969)
Valence Tautomerism
M. Gorlitz and H. Gunther, Tetrahedron, 25, 4467 (1969)
I. D. Gridnev, O. L. Tok, N. A. Gridneva, Y. N. Bubnov, and P. R. Schreiner, J. Am. Chem. Soc., 120, 1034 (1998).